Also just this page in general will give you more ideas then you can handle in one deck.
July 19, 2019 10:35 p.m.
Hey, Gonti, Lord of Luxury is a good mono black Commander.
He doesn't have an ability that makes opponent discard, but instead you can steal a card from an opponents library and cast it. This effect can compliment a discard theme. Gonti is good with Strionic Resonator which can also copy a triggered discard ability and there's many discard effects that are triggers.
Gonti is good with reanimation and/or recursion to take advantage of his ETB ability and deathtouch. Black has lots of reanimation or recursion options and these effects are helpful to be able to play cards that make all players discard such as Mindslicer and Oppression .
July 20, 2019 1:17 a.m.
Sidisi, Undead Vizier is a tutor on a body, though honestly mono black is kind of rough to pull off as there's essentially a limited number of strategies to play out for just one color, cabal coffers and stronghold for ramp tutors for days and much boring day.
Though it's not mono black or strictly discard strategy, my Crosis, why you make him discard?! has some interesting ways to keep opponent's hands stocked and discard engines to run. Storm Cauldron Noetic Scales and Warped Devotion all make for discard fodder making engines you can run in a mono black deck. And if you like creature-less you can run Pithing Needle naming Lethal Vapors , Temporal Extortion is also a weird Time Walk